page_title: Belemnitida
section_title: Mortality
hierarchical_section_title: Belemnitida / Paleoecology / Mortality
caption_attribution_description: English: Fossil of Hybodus, with belemnites in the stomach region.
context_page_description: Belemnitida is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most to the tip: the tongue-shaped pro-ostracum, the conical phragmocone, and the pointy guard. The calcitic guard is the most common belemnite remain. Belemnites, in life, are thought to have had 10 hooked arms and a pair of fins on the guard. The chitinous hooks were usually no bigger than 5 mm, though a belemnite could have had between 100 and 800 hooks in total, using them to stab and hold onto prey.
  Belemnites were an important food source for many Mesozoic marine creatures, both the adults and the planktonic juveniles, and likely played an important role in restructuring marine ecosystems after the Triassic–Jurassic extinction event. They may have laid between 100 and 1,000 eggs. Some species may have been adapted to speed and swam in the turbulent open ocean, whereas others resided in the calmer littoral zone and fed off the seafloor. The largest belemnite known, Megateuthis elliptica, had guards of 60 to 70 cm.
context_section_description: Belemnites were likely an abundant and important food source to many sea-going creatures of the Mesozoic. Belemnite hook remains have been found in the stomach contents of crocodilians, plesiosaurs, and ichthyosaurs; and the coprolite remains of ichthyosaurs and the extinct thylacocephalan crustaceans. Some animals may have only eaten the heads, leaving the phragmocone and guards, however, the guards of around 250 Acrocoelites were found in the stomach of a 1.6 m (5 ft 3 in) Hybodus shark, and a fragment in an Oxford Clay marine crocodile, meaning they were eaten whole. It may be that they were to regurgitate the indigestible matter later, similar to the modern sperm whale. To defend themselves, belemnites likely were able to eject a cloud of ink.
  The abundant planktonic belemnite larvae, along with planktonic ammonite larvae, likely formed the base of Mesozoic food webs, serving a greater ecological function than the adults. Giant pachycormid fish are thought to have been the main filter feeders of the time, occupying the same niche as modern baleen whales.
  Large accumulations of guards are commonly found and have been nicknamed "belemnite battlefields". The most quoted explanation is that belemnites were semelparous and died shortly after spawning, much like modern coleoids which migrate from the ocean to the shelf area. In battlefields comprising both adults and juveniles—as the former model would consist entirely of adults—large groups of belemnites may have been killed by volcanism, changes in salinity or temperature, harmful algal blooms (and, thereby, anoxia), or mass stranding. Another popular theory is that the guards were simply moved or redeposited by ocean currents into large aggregations. Some battlefields may be regurgitated indigestible matter from a predator.
caption: The shark Hybodus with belemnite guards in its stomach, State Museum of Natural History Stuttgart
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